User level threads without makecontext, getcontext, swapcontext

Is there any alternative to the context functions defined in <ucontext.h> that I can look into using? I had considered using setjmp and longjmp functions but I do not seem to have the ability to allocate a thread stack using setjmp and longjmp similar to how one can allocate a stack in makecontext

I ask because this is implemented in other places and people have gotten this to work across different platforms. I know that using makecontext, getcontext and swapcontext will not work on lots of operating systems like Mac OSXs